Optical storage can be used to achieve high areal density data storage, i.e., amount of data per unit recording area. The areal density of an optical storage device, in principle, is only limited by the diffraction limit of an illuminating optical beam for reading or writing. Electro-optical data storage systems based on magneto-optical materials may be configured to produce areal data densities of up to or greater than about one gigabit (i.e., 10.sup.9 bits) per square inch.
One approach to increase the areal data density in an optical storage system is to use a reduced beam size to write or read in a small spot. A monochromatic optical beam has a theoretical diffraction limit on the minimum beam size which is approximately on the order of its wavelength. Light sources with short wavelengths, such as those toward the blue end of the optical spectrum, may be used to further decrease the beam size and thereby achieve an even higher areal density.
Alternatively, the numerical aperture of the objective lens can be increased to focus a beam of a given wavelength to a reduced small spot within the diffraction limit. The reduced beam spot increases the areal density.